Optical interferometers are often used to make precise displacement measurements. For example, in semiconductor processing applications, a stage having a wafer mounted thereon must be positioned with respect to a projector that projects an image used in the lithographic processing of the wafer. The accuracy needed in this positioning operation depends on the feature size of the fabrication process being utilized; hence, as feature sizes have decreased, there is a need for ever more precise positioning mechanisms, and hence, devices for measuring the position of the stage.
In one class of interferometers, a light beam generated by a laser is split into two beams. One beam is reflected from the device that moves and whose position is to be measured, and the other beam is reflected from a reference mirror whose position does not change. The two beams are then combined and the interference patterns observed to provide a measurement of the change in position of the device. Typically, a mirror is placed on a surface of the device and the measurement laser beam traverses a path through the air from the interferometer to the mirror and back. The interferometer measures the change in optical path length between the reference beam and the measurement beam. The optical path length depends both on the distance from the interferometer to the device and the index of refraction of the material through which the beam travels. Hence, a change in the index of refraction cannot be distinguished from a change in position of the device being measured. The index of refraction of the air between the interferometer and the mirror can change due to many environmental factors such as temperature fluctuations, and hence, becomes problematic when very high precision measurements are required.
Accordingly, interferometer designs in which the light beam must traverse a significant distance in air have significant problems meeting the precision required in many applications. The laser interferometer described above requires a light path in air that is at least as long as the largest displacement of the stage being measured. Hence, paths of the order of 12 inches in air are required for the largest wafers currently used in semiconductor processing.
One method for avoiding long paths is to use a form of encoder in which the distance between the measurement device and the stage does not change significantly when the stage moves in the direction of measurement. Conventional optical encoders are examples of such devices. In such encoders, a pattern is attached to the stage such that the pattern moves past a detection head that measures the pattern by the reflection or transmission of a light beam. An encoder of this type is described in a co-pending US patent application 2007/0146722 filed on Dec. 23, 2005. However, providing an encoder of this type that can be reduced to a convenient package size has remained problematic for many applications.